MIDI Protocol - MIDI systems configuration

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To introduce this topic we need to define the terms master and slave. By master we mean a device that controls one or more slave devices. A master device can potentially control an infinite number of slave devices.

If the master device is a computer, and the slave devices have an USB port, all these setups can be made via USB. If the number of available USB ports are too few to plug all the slave devices, it is possible to gain more USB ports using a port replicator. The setups described in this section refer to classic MIDI systems, made up using MIDI connectors and MIDI cables.

20.3.1. Daisy chaining

Here we have a master device (a keyboard) and a set of slave devices in a cascade set up (ext1 and ext2 could be an electric drum-kit and keyboard module for example):

The Midi Protocol - Daisy Chain configuration diagram

Daisy Chain configuration diagram

Make sure you don't mistake the midi connections (in red) with the audio connections (in blue). The audio signals at each device's output are picked up and sent to the mixer to then be transformed into music. The MIDI OUT signals at the master's output is sent to the MIDI IN input of the Ext1 slave; a copy of this signal is also present on the MIDI THRU socket of Ext1 which thus gets sent to the Ext2 slave (for example, we can use channel 1 to pilot the Ext1 module and channel 2 to pilot the Ext2 module). This way the master pilots all the slave devices in cascade. The problem with this configuration is that the transferring process of the MIDI signal to each module's MIDI THRU socket introduces a slight delay. The devices are arranged in cascade and therefore all of the delays sum up, and if too many devices are present in the setup the overall synchronization of the devices will be at risk.

20.3.2. Daisy chaining with a sequencer

A sequencer is a device or a software which is capable of recording sequences of MIDI commands (one sequence refers here to one MIDI channel) and to subsequently reproduce them. For example, we could play a melody on a keyboard and record it onto a sequencer, then we could overdub it with another melody and so on. In the end, all the sequences we will have stored are run all together by the sequencer; in other words they are sent from the sequencer to the keyboard, and the whole piece will be re-played automatically.

The following diagram illustrates an example of daisy chaining configuration using a sequencer:

The Midi Protocol - Daisy Chain with sequencer configuration diagram

Daisy Chain with sequencer configuration diagram

The audio signals (blue) are, as previously, sent to the mixer. Let's take a look at the MIDI part: our sequencer is a computer equipped with a MIDI interface and a sequencing software. The MIDI OUT socket of the keyboard (which is still our master) is connected to the MIDI IN port of the computer's MIDI interface. This allows us to "record" the sequences played on the keyboard-synthesizer onto the computer. The computer's MIDI OUT socket is connected to the master's MIDI IN (still our keyboard) in order to re-play the recorded sequences. A copy of this signal is then sent, by means of the MIDI THRU socket, to devices Exp1 and Exp2. However, we still have the same problems with data-transfer delay.

In the next section we'll be taking a look at a configuration that will help us to resolve this problem.

20.3.3. MIDI THRU Splitter Box configuration

In any electrical setting a splitter is nothing more than a device which picks up an input signal and reproduces n copies of it at the output. An example of a splitter? Multiple adapter extension cables, such as those you get in households to have n number of current sockets out of a single socket. Splitters can either be active or passive. A passive splitter generally has very little circuitry, or in fact sometimes no circuitry whatsoever, and seeing that it doesn't reinforce the signal at all, it introduces a loss of signal every time it is split into two or more copies. So, it is impossible to create passive splitters with too many output sockets. This problem is resolved by using active splitters in which appropriate circuits reinforce the signal before it reaches the output sockets, thus bringing it back to the levels present in the input signal.

The following MIDI configuration shows the use of an active splitter which resolves the delay problems:

The Midi Protocol - MIDI splitter box configuration diagram

MIDI splitter box configuration diagram

In this case all the devices are connected to the splitter (which introduces a delay, but since it is the only one in the chain of connections it doesn't affect the synchronization of the devices).

20.3.4. Configuration with extended MIDI Interfaces

When more than 16 channels are needed we resort to MIDI interfaces with multiple outputs (MIDI OUT):

The Midi Protocol - Multiple Output MIDI Interfaces

Multiple Output MIDI Interfaces

In this case each MIDI output handles 16 channels. As we have already said, on the sequencing software a sequence will correspond to a MIDI channel and will also be referred to one of the MIDI outputs. In the diagram we can see that the electric drum-kit sequence will be associated to channel 1 (usually electric drum-kits only have 1 MIDI channel associating all the sounds identified by the various notes) of the MIDI 2 output.


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